This invention relates to a probe for testing logic signals and more particularly relates to such a device having minimum components for indicating whether a logic signal is at low, intermediate or high level or that an open circuit exists in a tested circuit.
Some prior art probes for testing logic signals have used a single indicator, which upon energization indicates occurrence of a high level electric signal and indicates the occurrence of a low level electric signal when not so energized. Such devices, however, do not provide a positive indication of occurrence of low level signals, nor are they capable of indicating occurrence of a signal in the deadband intermediate region relative to normally occurring low and high level signals.
Test devices for determining whether logic signals occurring in electric circuits are at low or high level, for example respectively representing logic 0 and logic 1, have utilized various types of indicators, such as lamps, light emitting diodes, and the like. Many of the prior art devices have been limited to indicating whether such logic signals are at low or high level without being capable of polarity reversal or providing an indication that a tested circuit is open or that the signal is at an intermediate level, the latter often implying a malfunction in the tested circuit. Moreover, those devices capable of three mode indication, viz, low intermediate and high level signals, have required complex and expensive circuitry to achieve the same and have had relatively high power requirements for energization, as, for example, the device disclosed in U.S. Pat. No. 3,525,939.
Liquid crystals in which the molecules become non-aligned upon application of an electric field at a potential equal to or greater than the threshold level of the liquid crystal have been used as indicators or display elements. In such arrangements, a light source may be located on one side of the liquid crystal, which either blocks or transmits light when de-energized and performs the opposite function when energized. Alternatively, such liquid crystals have been located in front of reflective surfaces, which provide for use of ambient light as the light source. An article in the October, 1973, issue of Fortune magazine entitled "What Makes Liquid Crystals Shine" presents some of the unique properties of liquid crystals, and particularly describes the use thereof in displays.
Although liquid crystals have a relatively high resistivity and require very low power for energization, one difficulty in using liquid crystal displays for indicating low level signals is that even though the threshold level for a liquid crystal can be determined during the manufacturing thereof, it becomes increasingly difficult and expensive to manufacture the same as the required threshold level is decreased. Low level logic signals, for example, in positive logic applications are often in the zero to 0.4 volt range, and it is often impossible to obtain a liquid crystal with such a low threshold level.